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The specific gravity of soft pitches is usually 1-250-1-265, that of hard
1-275-1-280, and that of very hard 1-275-1-280. For pitch from gas tar, the
carbon content is rarely less than 25-30%, and for that from vertical retorts
or blast-furnaces, 5-7%. The ash content is less than 0-5% for gas pitch and
more than i% (6-10%) for that from blast-furnaces. The yield of coke varies
from 30 to 60%, and the coke has a more or less porous appearance according
to the type of pitch from which it is derived (very porous with the very hard
pitches, less so with the others).

As regards the temperatures of softening and fusion, the following limits
may be taken for different types of pitch :

Soft:  Softens at 40°, melts at 50-60°.

Hard : Softens at 60°, melts at 70-80°.

Very hard : Softens at 80-85°, melts at 90-120°.

A good pitch for making briquettes should, according to Spilker 1 have
the following properties : not more than 0-5% of ash ; softening point between
60° and 75° ; solubility in aniline, 70-75%, and in carbon disulphide, not less,
than 70% ; yield of coke, 45% ; appearance of coke, caking and not too much


These are usually creosote oils or anthracene oils, freed more or less
completely from crystallisable substances, and are used for the impregna-
tion of wood, especially railway sleepers and telegraph poles, with the
object of preserving it.

In general they are brownish red or blackish liquids, more or less fluores-
cent, somewhat viscous, and with a more or less marked odour of the
products of tar distillation.

The principal tests to be made are :

1.  Determination of the Specific Gravity.—By means of a hydro-
meter or Westphal balance at 15° C.

In some cases measurements are made at higher temperatures, e.g., at 25,
45, 50° C. ; the temperature used must be indicated in the report.

2.  Distillation.—This is carried out in a tubulated retort of about
300 c.c. capacity, furnished with a thermometer.   The retort is charged
with loo c.c. of the liquid and the thermometer bulb arranged at about 2
cm. from the liquid, which is heated so that 120 drops per minute pass
over;  the different fractions are measured.

3.  Determination of the  Phenols  and  the  Naphthalene.—The
fractions obtained as under 2 are reunited in a graduated cylinder, shaken
repeatedly with 100 c.c. of caustic soda solution^(D 1*15) saturated with
sodium chloride and then left to settle ; the increase in volume of the soda
solution gives the percentage of phenols.   In the supernatant oily layer
the naphthalene is determined by cooling (to 15°) in the manner indicated
for middle oils.

4.  Test for the Presence of Solid  Substances.—20 c.c. should
remain liquid when heated to 40° and when shaken with 20 c.c. of pure
benzene:  when filtered through paper, the solution thus obtained should
not leave a brown mark on the filter.

1 Lunge:  Coal-Tar and Ammonia (London, 1916), p. 542.